There have been known electrophotographic image forming apparatuses such as multifunction printers, copying machines, printers, and facsimile machines. An electrophotographic image forming apparatus is configured to (i) form an electrostatic latent image on a surface of a photoreceptor, (ii) cause a developing device to supply toner to the electrostatic latent image so that this image is developed to form a toner image, and (iii) transfer the toner image onto a sheet so as to fix the toner image on the sheet.
In recent years, an image forming apparatus capable of forming high-quality color images frequently uses a two-component developer (hereinafter also referred to simply as “developer”), which has an excellent stability in charging of toner. The developer includes toner and carrier, which are stirred in the developing device to cause friction between them. This friction allows properly charged toner to be obtained. The electrically charged toner is supplied onto a surface of a developing roller (developer bearing member). The toner on the surface of the developing roller is electrostatically attracted onto an electrostatic latent image formed on a photoreceptor. This forms a toner image on the photoreceptor.
Recently, there has been a demand for a high-speed, downsized image forming apparatus. This has in turn necessitated rapid supply of developer onto a developing roller. To charge developer rapidly and sufficiently, and also to supply the developer onto a developing roller rapidly, a developing device of a circulation type is preferably used. An example of a developing device of the circulation type is disclosed in Patent Literature 1 cited below.
In a developing device including a developing roller which rotates so that its outer surface moves from below in an upward direction while facing a photoreceptor, among gaps between the developing roller and a case (developing tank) containing the developing roller, gaps located respectively at opposite ends of the developing roller which ends are located along an axis direction of the developing roller are blocked by a sealing material. Further, a gap below the developing roller is blocked by developer on a surface of the developing roller. However, among the respective gaps, a gap above the developing roller provides an opening section for communicating inside of the developing device with outside thereof. This is because the developer above the developing roller is smaller in amount than the developer below the developing roller due to a developing process. Rotation of the developing roller causes an airflow through the opening section, thus flowing outside air through the opening section into the inside of the developing device. In view of this, the developing device normally has a vent, as disclosed in Patent Literature 1 cited below, formed so as to cause such outside air flown through the opening section to be discharged to the outside of the developing device. Further, the vent is provided with a filter for preventing toner inside the developing device from scattering to the outside.